Rimmed steel has been used for a long period of time due to its excellent surface property for obtaining beautiful finishing of coating. The rimmed steel has an ageing property at room temperature due to the presence of nitrogen solid solved therein, and when the rimmed steel is press molded just after the cold rolling, the yield strength is increased due to the strain ageing by nitrogen without generation of stretcher-strain at the bake coating.
Recently, continuous casting technic of steel has been progressed and developed, and a large amount of aluminum killed steel having a moldability superior to that of rimmed steel is used in place of rimmed steel as a thin steel sheet for drawing, which is mainly used for exterior automotive plate.
Aluminum killed thin steel sheet is excellent in the deep drawing property, but is generally poor in the baking hardenability due to the presence of nitrogen fixed by aluminum.
While, when it is intended to use high tensile strength steel sheet in order to produce automobiles having a light weight, it is necessary to give baking hardenability, particularly an improved baking hardenability, to the high tensile strength steel sheet in order to compensate the decreasing of the dent resistance due to the decreasing of the sheet thickness from the view point of safety.
Ferrite-martensite dual phase steel sheet has a satisfactorily high baking hardenability, but has generally a low r value of about 1.0, and is poor in the drawability. Therefore, the use field of ferrite-martensite dual phase steel sheet is limited.
While, in order to produce a thin steel sheet having a high r value, there have been proposed the following treatments. That is, aluminum killed cold rolled steel sheet, which has been strengthened by adding phosphorus thereto, is subjected to an open coil annealing and solid solved carbon is left in the steel to develop the strain ageing property by utilizing such a property that the open coil annealed steel can be cooled at a rapid cooling rate. Alternatively, the aluminum killed cold rolled steel sheet is subjected to a tight coil annealing at a particularly high temperature to form coarse carbide and to disturb the precipitation of solid solved carbon, whereby solid solved carbon is left in the steel (for example, refer to Iron and Steel, Vol. 66, page A209 (1980)). However, in the former method, additional treating steps, wherein the steel sheet is rewound into an open coil and into a tight coil before and after annealing respectively, must be carried out. While, in the latter method, fusing of the adjacent layers of the coiled steel sheet occurs and further the inner cover (retort) of the annealing furnace is thermally deformed. Therefore, the production cost is very high. Moreover, it has been found that the phosphoruscontaining low carbon aluminum killed steel sheet, that is, so-called rephos steel, which has been subjected to the above described treatments, is not always satisfactory in the r value and yield strength.
While, a steel sheet having a high r value and a low yield point, which is produced by adding Ti, Nb and the like to extra low carbon steel to fix C and N, and by adding P, and the like thereto to form a solid solution and to strengthen the steel, is used in the automotive parts more widely than the above described steel. However, the steel has a low yield strength and a high tensile strength, and therefore when plastic strain is applied to the steel, the steel has a remarkably high hardenability in the working. However, it is impossible to cause uniform plastic deformation over an entire working range of the molding by a press mold depending upon the shape of parts produced by the molding. Accordingly, the portion, to which a low plastic strain is applied, still has a low yield strength, and is easily deformed by a small external force.
In order to obviate the above described drawbacks, there have been attempts to give the baking hardenability to such steel. That is, Japanese Patent Laid-Open Application No. 114,717/78 discloses Ti addition, Japanese Patent Application Publication No. 30,528/76 discloses Zr addition and Japanese Patent Laid-Open Application No. 130,819/74 discloses Nb addition. In all these methods, Ti, Zr and the like are contained in a steel in an amount a little smaller than the amount of C+N in order that C and N in the steel are not completely fixed but solid solved C and N are left in the steel in an amount not to cause deterioration of deep drawing property while directing to the prevention of ageing at room temperature, and further the steel is cooled at a cooling rate, which does not cause carbide and nitride of iron precipitated in the relatively low temperature region in the cooling step after the annealing.
However, even in these methods, a little amount of solid solved C and N is always contained in the steel sheet before the cold rolling and at the rovery-recrystallization step after annealing. Therefore, the steel has a serious drawback that the development of aggregation structure suitable for r is hindered. Therefore, it has been difficult to give a baking hardenability to the steel while maintaining a high r value.
For example, as to Nb addition, according to the above described Japanese Patent Laid-Open Application No. 130,819/74, and Nb-containing steel, which contains, in % by weight, 0.004% of C, 0.03% of Al and 0.062% of Nb, is hot rolled, continuously annealed at a uniform temperature of 800.degree. C., whereby a steel sheet having an age hardening value of 17.8 kg/cm.sup.2 is obtained (by treatment of prestraining under 3% tension and then artificial ageing treatment at 200.degree. C. for 30 minutes). However, the r value is only about 1.71, and further the amount of Nb is excessively large as compared with the amount of C, and the steel sheet is low in the elongation and is not satisfactory in the ductility.
The inventors have already found out that, when a steel sheet containing Nb in an amount smaller than the equivalent amount, calculated as Nb, to the amount of C is hot rolled at high speed and at high reduction rate, a steel sheet having non-ageing property, deep drawing property and more improved ductility can be obtained; and further found out that the use of phosphorus as a solid solution strengthening element under the above described condition is effective for improving the deep drawing property without adverse influence upon the r value. Based on the experience, the inventors have further studied how to give baking hardenability to steel and how to improve the baking hardenability and reached the present invention described hereinafter.